1. Field of the Invention
The present invention relates generally to an antenna, and more particularly to a small broadband monopole antenna including a shorted patch and a probe with a strip line that are electromagnetically coupled with each other. The probe with the strip line has a length of about λ/4, where λ is a wavelength.
2. Description of Prior Art
Recently, the wireless communication system has been diversely and rapidly developed into a cellular phone, a personal communication service (PCS), an international mobile telecommunication-2000 (IMT-2000), and a personal digital assistant (PDA) and its market also has been enlarged to provide services at a high speed. In the IMT-2000, which is also called a third generation mobile communication system, and to which a great deal of research and development have been done, diverse communication services are available not only for voice and low speed data but also for high speed multimedia data. Together with the developments of such a variety of mobile communication systems, many efforts have been also made to develop small personal portable communication terminals with a high performance. For the miniaturization of the communication terminals, it is commonly regarded that the embedded type small antenna is essential.
Commonly, the prior communication terminals widely used an external type retractable antenna such as a helical antenna or a monopole antenna. However, the external type retractable antenna is disadvantageous for the miniaturization of the communication terminals. A planar inverted F antenna (PIFA) and a short-circuit microstrip antenna are suggested as a small embedded antenna to replace the external type retractable antenna.
These antenna structures have a benefit of a simple design, but unfortunately have a narrow bandwidth. In order to improve the narrow bandwidth problem of the PIFA and the short-circuit microstrip antenna, several types of antennas are suggested such as a 2-lines type normal mode helical antenna (NMHA), a meander line antenna consisting of two strips, a double line PIFA antenna, and a PIFA with stacked parasitic elements. These antennas are detailed in the following: 1) K. Noguchi, M. Misusawa, T. Yamaguchi, and Y. Okumura, “Increasing the Bandwidth of a Meander Line Antenna Consisting of Two Strips,” IEEE AP-S Int Symp. Digest, pp. 2198-2201, vol. 4, Montreal, Canada, July 1997; 2) K. Noguchi, M. Misusawa, M. Nkahama, T. Yamaguchi, Y. Okumura, and S. Betsudan, “Increasing the Bandwidth of a Normal Mode Helical Antenna Consisting of Two Strips,” IEEE AP-S Int Symp., pp. 782–785, vol. 2, Atlanta, USA, June 1998; 3) M. Olmos, H. D. Hristov, and R. Feick, “Inverted-F Antennas with Wideband Match Performance,” Electron. Lett., vol. 16, no. 38, pp. 845–847, August 2002; and 4) S. Sakai and H. Arai, “Directivity Gain Enhancement of Small Antenna by Parasitic Patch,” IEEE AP-S Int. Symp., pp. 320–323, vol. 1, Atlanta, USA, June 1998. Among these antennas, the meander line antenna can have wider bandwidth than that of the 2-lines type NMHA or the PIFA by offsetting a balanced mode (transmission line mode) with an unbalanced mode (radiation mode).
Other solutions for obtaining a wide bandwidth include a method of attaching a patch with a shorting wall to an L-strip feed or an L-prove feed and a method of electromagnetically coupling the PIFA with the shorted parasitic patch. These solutions are detailed in the following: 1) C. L. Lee, B. L. Ooi, M. S. Leong, P. S. Kooi, and T. S. Yeo, “A Novel Coupled Fed Small Antenna,” Asia-Pacific Microwave Conf., pp. 1044–1047, vol. 3, Taipei, Taiwan, December 2001; 2) Y. X. Gou, K. M. Luk, and, K. F. Lee, “L-Probe Proximity-Fed Short-Circuited Patch Antennas,” Electron. Lett., vol. 24, no. 35, pp. 2069–2070, November 1999; and 3) Y. J. Wang, C. K. Lee, W. J. Koh, and Y. B. Gan, “Design of Small and Broad-Band Internal Antennas for IMT-2000 Mobile Handsets,” IEEE Trans. Microwave Theory Tech., vol. 49, no. 8, August 2001. These antenna structures can satisfy with a bandwidth of 30% or more, but has have some restrictions in reducing antenna size since because the L-strip structure and a shorted patch should have a resonance length of about λ/4.
For example, U.S. Pat. No. 6,452,558 entitled “Antenna Apparatus and a Portable Wireless Communication Apparatus” discloses a diversity antenna constructed by contacting a planar inverted F antenna (PIFA) with a monopole antenna. The diversity antenna uses two receiving antennas to create two paths for receiving electromagnetic waves in order reduce a fading phenomenon.
As another example, U.S. Pat. No. 5,289,198 entitled “Double-Folded Monopole Antenna” discloses an antenna that is constructed by folding a wire monopole antenna. This antenna has a total length equal to 1.0 λ of a resonance frequency and uses a traveling wave for its operation. The antenna does not use electromagnetic coupling with the shorted patch.
In addition, Korean Patent Application No. 10-2001-7000246 (with a U.S. counterpart application Ser. No. 09/112,366 filed on Jul. 9, 1998), entitled “Small Printed Spiral Type Antenna for Mobile Communication Terminals”, discloses an antenna structure of a spiral type monopole antenna and uses a method of directly connecting a grounding post to the spiral type monopole antenna to achieve an impedance matching. However, these antennas have different structures and characteristics from the antenna according to the present invention as will be described below.